Really the only difference in gel frames is the 7.5" frames used for ETC pars. They are cut a bit larger which lets a bit more light through. But you can mix and match 10" frames, 7.5" frames, and 6.25 frames as you please, just keep in mind the ETCpar thing.
The rule of thumb regarding lamp sizes does not apply to gel frames, as it is completely dependant upon the maker of the PAR or Fresnel. While you can use the formula for figuring bulb diameters, you cannot accurately find the gel frame size.
PAR 16 is 3.0 inch gel frame PAR 20 is 3.25 inch gel frame - 2.375 inch opening PAR 30 is 4.375 inch PAR 36 is 6.25 inch PAR 38 is either 6 inch or 7.50 inch gel frame PAR 46 is either 6.25 inch or 7.5 inch PAR 56 is either 7.5 inch or (MBT) 9 inch PAR 64 is 10.0 inch
Also- mostgel frames are square, while budget frames are hexagonal. The yield per sheet of aluminum/sheet metal is higher when arranging for this odd size.